With existing tire polymer and compound technology there is a tradeoff between desirable tire properties using commonly used tire polymers. The impending introduction of tire labeling regulations, along with increased competition make it more important than ever to produce tires that show high performance in every category. Tire properties are directly related to the material properties of the polymers used in the tire, which are in turn directly related to the glass transition temperature (Tg) of the chosen polymer. The most commonly used tire polymers are cis-polybutadiene, natural rubber, high-vinyl polybutadiene, and styrene/butadiene copolymers; these polymers are largely chosen for different roles in tire compounds based on their Tg. The traditional relationships between polymer Tg and three important tire performance properties are outlined in the following table.
Performance characteristicEffect of increasing TgRolling resistanceworsenedTread wearworsenedWet tractionimproved
This relationship between Tg, tan delta, and tire properties can also be visualized by examining the tan delta curves of various polymers. The value of tan delta at various temperatures is commonly used as an indicator of polymer performance, for example, the tan delta at 0° C. is an indicator of wet performance, while the tan delta at 60° C. is an indicator of rolling resistance. A tire with a higher tan delta at 0° C., such as an SBR, also exhibits a high tan delta at 60° C., making it a good choice for wet performance, but bad for rolling resistance. The opposite is true for cis-polybutadiene: the tan delta is low at both 0° C. and 60° C., making cis-PBD an excellent choice for improved rolling resistance but poor for wet traction performance.
There is a tradeoff between wet performance and the other two important characteristics, and it would seem at first glance that there is no way around this natural tradeoff. However, the shape of the tan delta/temperature curve also influences the polymer properties. Polymers of different composition exhibit different relationships between their glass transition temperatures and physical properties. An ideal polymer would exhibit a higher Tg and a steeper tan delta/temperature slope, allowing tan delta to be as high as possible in the wet traction regime and as low as possible in the rolling resistance regime. This ideal polymer is not known among existing tire elastomers, therefore there is a need for new technology to prepare new classes of polymers with properties approaching those of this ideal polymer.
A largely unexplored class of polymers includes high-cis copolymers of dienes with other substituted comonomers beyond butadiene, isoprene, or styrene. One of the first reports of Ziegler/Natta copolymerization of butadiene (BD) with cyclohexadiene (CHD) is reported in GB 1294167. The catalyst in this system was an allyl complex of nickel, and the resulting highly cis copolymers contained between 5-25% incorporation of cyclic comonomers. Later reports of Ziegler/Natta copolymers of CHD and BD are given in U.S. Pat. No. 4,113,930, U.S. Pat. No. 4,179,480, and U.S. Pat. No. 4,223,116. The processes described in these patents employ transition metal catalysts, such as bis(1,5-cyclooctadiene)-nickel for the copolymerization of butadiene with cyclohexadiene. The copolymers described in these reports generally contained a high percentage of the cis-1,4 microstructure (>90%), however increasing the comonomer content of the feed led to reduced conversion and yield. In one report of this series, the authors found that the resulting cyclohexene/butadiene copolymers blended with SBR polymers showed improved green strength over the SBR alone.
More recently, WO 2011/04702 discloses a family of high-cis polybutadiene/cyclic diene copolymers of up to 5% CHD content. The publication discloses a variety of transition metal and lanthanide catalysts, however the representative examples focus on a nickel-based catalyst mixture.
The cyclohexene content of the CHD/BD copolymers described in WO 2011/04702 was only found to be approximately 60% of the expected content based on the feed ratios, and the polymer yield dropped noticeably with increased cyclohexadiene content. The glass transition temperatures, melting points, and PDI of the copolymers were not significantly changed from those of the butadiene homopolymers, likely due to the low incorporation of CHD.
Another diene, 1-vinylcyclohexene, remains relatively unexplored as a monomer in Ziegler/Natta polymerizations.
One example of Ziegler/Natta polymerization of 1-vinylcyclohexadiene was published in Longo et al., Macromol. Rapid Commun. 1998, 19, 229. Highly cis-1,4 polymer can be prepared from 1-vinylcyclohexene using a cyclopentadienyl titanium trichloride/MAO catalyst system. In this same report, 1-VCH was polymerized with ansa-metallocene zirconium compounds to form polymers of a 1,2-structure. Cationic polymerization of the same monomer was reported in Hara et al., J. Polym. Sci. A 1971, 9, 2933.